docs: def _compute_diff(self):
          """Shows a diff between this version and the previous version"""
          history = self.env['document.page.history']
          for rec in self:
              domain = [
                  ('page_id', '=', rec.page_id.id),
                  ('state', '=', 'approved')]
              if rec.approved_date:
                  domain.append(('approved_date', '<', rec.approved_date))
              prev = history.search(domain, limit=1, order='approved_date DESC')
              if prev:
                  rec.diff = self.getDiff(prev.id, rec.id)
              else:
                  rec.diff = self.getDiff(False, rec.id)